1. Field of the Invention
The invention relates to a backlight unit, and more particularly, to a backlight unit having a supplementary lamp for a smooth starting operation.
2. Discussion of the Related Art
A cathode ray tube (CRT), a type of flat display device, has been widely used in various applications that include television monitors, measuring machines and information terminals. However, CRTs cannot satisfy demands for miniaturization and weight requirements due to their large size and relatively high weight. Therefore, various types of alternative technologies have been successfully used as substitutes for CRTs including: liquid crystal displays (LCDs), which use an electric field optical effect, plasma display panels (PDPs) which use gas discharge, and electroluminescence display devices (ELDs), which use an electric field luminous effect.
Among the various display devices, LCD devices have been most actively studied and developed because of their advantageous characteristics such as compact size, low weight, and low power consumption. LCDs have been used in many applications that require ultra-thin flat display devices, for example, notebook computers, laptop computers, desktop computers and other types of large display devices. Because of these various applications, the demand for the LCD devices continuously increases.
Most LCD devices operate by controlling the light transmittance to the display image. Accordingly, it is necessary to provide a light source such as a backlight unit in an LCD panel. In general, the backlight unit used in LCD devices has a cylindrical light-emitting lamp, which is classified into two types according to the arrangement of the light-emitting lamps: 1) a direct type backlight unit or 2) an edge type backlight unit.
In edge type backlight units, a lamp unit is provided at one side of a light-guiding plate, with the lamp unit being provided with a lamp that emits light. Additionally, a lamp holder is inserted into both ends of the lamp to protect the lamp, and a reflective sheet is provided with one side of the sheet inserted into the light-guiding plate and surrounding the circumference of the lamp. The sheet reflects the light emitted from the lamp toward the light-guiding plate. Because of this structure, edge type backlight units are generally used in relatively small sized LCD devices such as monitors for laptop computers and desktop computers. Edge type backlight units are advantageous in these applications because they allow the LCD unit to feature great light uniformity, a long life span, and a thin profile.
With the trend toward using larger sized LCD devices of 20-inches or more, the direct type backlight unit is being actively developed. Direct type backlight units include multiple lamps that are formed in parallel on a lower surface of a light-diffusion sheet, whereby an entire surface of the LCD panel is directly illuminated with light. The direct type backlight unit has greater light efficiency as compared with the edge type backlight unit, and is therefore often used with large-sized LCD devices that require high luminance. For example, direct type backlight units are often used for large-sized monitors or televisions. These units are often operated for extended periods of time and often have long service lives, which often causes lamps to burn out or otherwise become inoperable.
The loss of one lamp in an edge type backlight unit only causes a slight decrease in luminance on the LCD screen. As discussed above, direct type LCD devices are provided with multiple lamps directly under the screen of the LCD panel. Accordingly, if one of the lamps burns out or becomes inoperable, the portion of the LCD screen corresponding to the lost lamp is darker than the surrounding portions of the screen.
The edge and direct type LCD devices may use any of the following types of light sources: Electro Luminescence (EL), Light Emitting Diode (LED), Cold Cathode Fluorescent Lamp (CCFL), Hot Cathode Fluorescent Lamp (HCFL) or External Electrode Light-emitting Lamp (EEFL).
Hereinafter, a related art direct type backlight unit will be described with reference to the accompanying drawings.
FIG. 1 shows a schematic view of a direct type backlight unit according to the related art.
As shown in FIG. 1, a related art direct type backlight unit includes multiple light-emitting lamps 1, an outer case 3, and light-scattering devices 5a, 5b and 5c. In this case, the outer case 3 fixes and supports the plurality of light-emitting lamps 1, and the light-scattering devices 5a, 5b and 5c are provided between the light-emitting lamps 1 and an LCD panel (not shown).
The light-scattering devices 5a, 5b and 5c are formed of multiple diffusion sheets and one diffusion plate and prevent the silhouette of the light-emitting lamps 1 from being shown on a display surface of the LCD panel (not shown). This structure and orientation provides a light source with uniform luminance. Also, a reflective sheet 7 is provided inside the outer case 3 for concentrating the light emitted from the light-emitting lamps 1 to the display part of the LCD panel, which improves the efficiency of the unit.
In one aspect of the related art, each light-emitting lamp 1 is formed from a Cold Cathode Fluorescent Lamp (CCFL). Also, electrodes are provided at each end of each CCFL, and the CCFL emits light when electrical power is provided to the electrodes. The ends of each of the light-emitting lamps 1 are fixed to the sides of the outer case 3. Power supplying wires 9 and 9a are connected to the electrodes 2 and 2a that are provided at the ends of each of the light-emitting lamps 1 to transmit power to the light-emitting lamps 1. The power supplying wires 9 and 9a are connected to a driving circuit by an additional connector. Each light-emitting lamp 1 requires an individual connector.
That is, the power supplying wire 9 connected to one electrode of the fluorescent lamp and the power supplying wire 9a connected to the other electrode of the fluorescent lamp are connected together to one connector. Any one of the power supplying wires 9 and 9a is curved around the lower side of the outer case 3, and then connected to the connector.
In the above explanation, the lamp is a CCFL. However, it is possible use an EEFL in the backlight unit.
If the backlight unit using the CCFL or EEFL is maintained under the dark state having a luminous intensity of 0.1(Lux) or less for a long time (that is, non-operating state), the gas of the lamp is in the ground state. Accordingly, when trying to operate the backlight unit, it is difficult to quickly and uniformly operate the backlight unit.